Linear particle accelerator system having improved beam alignment and method of operation

ABSTRACT

A linear particle accelerator having detection apparatus for detecting the presence of and correcting for beam misalignment. The linear accelerator includes a charged particle accelerator system and deflection coils for changing both the positional and angular displacement of a charged particle beam. A target is disposed in the particle beam path for emitting X-rays upon being struck by the charged particles. The photon field pattern developed by the target takes the form of a forward-peaked lobe configuration extending from the target. An arrangement of radiation responsive electrodes is disposed in the radiation field for developing electrical signals responsive to changes in the lobe pattern. The signals developed by the radiation responsive electrodes are applied to differential servo circuitry for applying signals to the deflection coils to correct for both positional and axial misalignment of the particle beam.

United States Patent [191 McIntyre et al.

[ Sept. 24, 1974 LINEAR PARTICLE ACCELERATOR SYSTEM HAVING IMPROVED BEAM ALIGNMENT AND METHOD OF OPERATION [75] Inventors: Raymond D. McIntyre; Craig S.

Nunan, both of Los Altos Hills,

Primary ExaminerJames W. Lawrence Assistant Examiner-Davis L. Willis Attorney, Agent, or Firm-Stanley Z. Cole; John .I. Morrissey 5 7 ABSTRACT A linear particle accelerator having detection apparatus for detecting the presence of and correcting for beam misalignment. The linear accelerator includes a charged particle accelerator system and deflection coils for changing both the positional and angular displacement of a charged particle beam. A target is disposed in the particle beam path for emitting X-rays upon being struck by the charged particles. The photon field pattern developed by the target takes the form of a forward-peaked lobe configuration extending from the target. An arrangement of radiation responsive electrodes is disposed in the radiation field for developing electrical signals responsive to changes in the lobe pattern. The signals developed by the radiation responsive electrodes are applied to differential servo circuitry for applying signals to the deflection coils to correct for both positional and axial misalign- POSITION ERROR SYM. SERVO COlLS-28 Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif.

[22] Filed: Feb. 26, 1973 [21] App]. No.: 335,634

[52] US. Cl 250/385, 250/397, 250/399 [51] Int. Cl. GOlt 1/18 [58] Field of Search 250/396, 397, 398, 399, 250/320, 374, 382, 384, 385, 389; 313/93, 55; 324/71 EB [56] References Cited UNITED STATES PATENTS 2,640,948 6/1953 BurrilL. 250/492 X 3,293,429 12/1966 LeBoutet et al. 324/71 EB 3,360,647 12/1967 Avery 313/55 X 3,373,283 3/1968 Lansiart et al. 250/389 ANGULAR ERROR I0 SYM. SERVO COILS-24 zxv 'v' 22 CHARGED PARTICLE BEAM ACCELERATOR TRANSPORT SYSTEM SYSTEM-26 EKXOYQYQI was 30 PATENTEU SEPZ 41974 SWEET 10$ 2 POSITION ERROR SYM. SERVO COILS-28 ANGULAR ERROR sYRsERvo COILS- Q' 'Q". v

CHARGED PARTICLE ACCELERATOR SYSTEM PATENTEI] 39241974 sum 20$ 2 FIG. 4

FIG. 5

LINEAR PARTICLE ACCELERATOR SYSTEM HAVING IMPROVED BEAM ALIGNMENT AND METHOD OF OPERATION CROSS REFERENCES TO RELATED PATENTS AND PATENT APPLICATION US. Pat. No. 3,322,950 to J. S. Bailey et al, entitled Linear Accelerator Radiotherapy Device and Associated Beam Defining Structure, issued May 30, i967 and assigned to the same assignee as the present invention.

U.S. Pat. No. 3,360,647 to R. T. Avery, entitled Electron Accelerator With Specific Deflecting Magnet Structure and X-ray Target, issued Dec. 26, 1967 and assigned to the same assignee as the present invention.

US. Pat. application Ser. No. 335,633 by R. D. Mc- Intyre, entitled Transmission Ionization Chamber, filed concurrently herewith and assigned to the assignee of the present application.

BACKGROUND OF THE INVENTION This invention pertains to the art of particle accelerators, and more particularly, to linear particle accelerators for use with high-energy X-ray systems, such as those used in X-ray therapy.

Modern methods of treating cancer and other related diseases demand high intensity levels of radiation for deep X-ray therapy applications. Therefore, high energy radiotherapy devices operate typically in a range of 4 to 25 million electron volts in order to obtain the desired radiation intensity distribution. In X-ray therapy it is necessary that the radiation be very precisely directed in order to obtain maximum clinical benefit from the high energy radiation.

Present day high energy X-ray systems generally comprise a charged particle accelerator which forms and projects a beam of charged particles onto a target for generating X-rays. The accelerated particles are fo cused and in some cases bent at 90 prior to being directed toward a target.

A heavy metal primary collimator is generally located at the downstream side of the target and is used to obtain the desired X-ray beam configuration. A fiattening filter and an ionization chamber are normally positioned in the X-ray beam to measure dose rate and to integrate the total dose in order to obtain uniform intensity of the beam across a plane normal to the beam path. An example of such a high-energy X-ray system is disclosed in the aforementioned United States patent to J. S. Bailey et al.

If the beam of charged particles strikes the target at a point displaced from the center of the target, the resulting pattern of emitted X-rays is correspondingly displaced. Also, if the angle of incidence at which the particle beam strikes the target is changed, there is an angular change in the radiation pattern of X-rays leaving the target. With positional and angular displacement of the X-ray pattern resulting from positional and angular misalignment of the charged particle beam striking the target, it is difficult, if not impossible, to accurately direct the emitted X-rays.

Half-plate or quadrant radiation responsive electrodes placed in an ionization chamber have been used to measure the distribution of radiation intensity across a charged particle radiation field. Generally, the quadrant electrodes have been symmetrically placed about a centerlineof the ion chamber. The ion chamber is then positioned so that its centerline is coincident with the central axis of a radiation field. Such an array of four electrodes will provide signals proportional to the 5 integral of radiation flux passing through each quadrant and may be used to monitor symmetry of the radiation field pattern about the central axis.

With certain conditions of beam misalignment, this quadrant arrangement has been found to be unsatisfac- 0 tory in detecting assymetry of the field at a plane remote from the ion chamber itself. For example, it is possible to have both a positional displacement of the charged particle beam on the target in combination with a change in the angle at which the beam strikes the target. In previous high-energy X-ray systems of the type which includes a charged particle beam impinging on a target, a heavy metal collimator located downstream of the target, and a field flattening filter and ion chamber, the charged particle beam may be controlled in such a manner as to cause the quadrant electrodes to indicate a balanced condition: however, the balanced field would only exist in the plane of the ion chamber and not at other planes remote from the ion chamber.

SUMMARY OF THE INVENTION The present invention is directed toward a radiation responsive detection system for controlling the alignment of a charged particle beam in order to maintain a desired radiation pattern, thereby overcoming the noted disadvantages, and others, of previous systems.

Based upon the principle that the radiation or photon field generated by a target being struck by charged particles will take the form of a forward-peak lobe pattern, it has been found that a slight change in the angle of incidence of the particle beam on the target results in a substantial tilt of this radiation lobe pattern. It has also been determined that a change in the position on the target at which the beam of particles strikes a target results in a corresponding displacement of the lobe pattern.

Accordingly, it has been found that by placing an outer set of radiation responsive electrodes at positions to monitor the extreme edges, or shoulders, of the photon field, the tilt of the lobe may be measured with a high degree of accuracy. A separate set of inner electrodes are then placed to measure radiation passing through the steepest slope of the flatness filter in order to detect changes in position of the lobe. Changes in the photon field due to tilt of the lobe at the inner portions of the lobe are compensated for by increased absorption in the flatness filter; therefore, the inner set of electrodes are unresponsive to changes in the tilt of the lobe. Thus, the inner electrodes provide only an indication of positional changes of the lobe.

In one aspect of the present invention, there is the provision of a particle accelerator which includes apparatus for forming and projecting a beam of particles into a substantially linear path. The linear accelerator also includes means for deflecting the particle beam, such as angular error symmetry servomechanism coils and positional error symmetry servomechanism coils. A target is disposed in the path of the particle beam for emitting X-rays upon being struck by particles. A radiation detector arrangement is placed in the radiation field for measuring certain parameters of the pattern of radiation, and a control circuit is coupled between the radiation detector and the beam deflector for correcting for angular and positional displacement of the particle beam.

In another aspect of the present invention, the detector arrangement includes radiation responsive electrodes for monitoring radiation intensity at the outer edges of the radiation field and electrodes positioned to monitor radiation intensity at positions close to an axis of symmetry of the radiation field in order to develop signals representative of changes in two parameters of the radiation pattern.

In another aspect of the present invention, there is the provision of an inner set of four electrodes which are each disposed in one of the quadrants about an axis of symmetry of the radiation field, and an outer set of four electrodes which are disposed more distant from the axis than the inner electrodes and each positioned in one of the quadrants.

In another aspect of the present invention, there is the provision of a method of aligning a charged particle beam, both with respect to angular changes of the longitudinal axis and displacement changes of the beam, by monitoring the tilt and changes in the position of a lobe radiation pattern which is formed when the particles strike a target.

The objects and advantages of the invention will become apparent from the following description of a preferred embodiment of the invention as read in conjunction with the accompanying drawings and in which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram, schematic view illustrating in basic form a high energy X-ray sysem incorporating beam alignment apparatus of the present invention;

FIG. 2 is a schematic view illustrating forwardpeaked lobe patterns resulting from changes in the alignment of the charged particle beam;

FIG. 3 is a plan view illustrating an arrangement of electrodes for monitoring the radiation pattern;

FIG. 4 is a sectional view of the electrode assembly of FIG. 3 taken along lines 44, and associated electrical servomechanism circuitry; and

FIG. 5 is a sectional view of the electrode assembly illustrated in FIG. 3 taken along lines 5-5, and associated electrical servomechanism circuitry.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 generally illustrates a high energy X-ray system comprising a particle accelerator system for accelerating and projecting charged particles onto a target 12. Upon being struck by the charged particles, the target 12 emits high energy X-rays.

Downstream from the target 12 is a heavy metal primary collimator 14 which is used to obtain the desired X-ray beam configuration. Positioned downstream of the primary collimator l4, and aligned with the open ing in the collimator, is a flattening filter 16 and an ionization chamber 18. The ionization chamber 18 is disposed in the radiation field to measure dose rate and for integrating the total radiation in order to generate electrical signals which are, in turn, used to maintain alignment of the particle beam. A jaw-shaped movable collimator 20 is positioned downstream from the flattening filter and ionization chamber for varying the radiation field size.

More particularly, the particle accelerator system 10 includes a charged particle accelerator 22 for forming charged particles, accelerating the particles, and focusing the particles into a beam. Angular error symmetry servomechanism coils 24, which are associated with the particle accelerator 22, primarily serve the function of changing the angle of incidence at which the beam of charged particles strike the target 12.

The beam of particles generated by the accelerator 22 passes through a beam transport system 26 and through position error symmetry servomechanism coils 28. The beam transport system 26 includes deflecting and focusing coils and various slits for shaping the particle beam. The position error servomechanism coils 28 primarily serve the function of changing the location on the target 12 at which the particle beam strikes the target.

Disposed between the position servomechanism coils 28 and the target 12 is a beam bending magnet 30 which serves to bend the beam of charged particles through a angle. Reference is made to the aforementioned United States Patent to R. T. Avery for an example of an electron accelerator which utilizes 90 beam bending techniques. It should be noted, however, that the practice of the present invention does not require such bending of the beam.

FIG. 2 illustrates the general configuration of a photon field generated at the target 12 when the target is struck by charged particles. For purposes of illustration, the photon field, which takes the form of a forward-peaked lobe pattern 36, is illustrated in a separate figure; however, it is to be understood that such a lobe pattern would actually exist in the X-ray system of FIG. 1.

As illustrated in FIG. 2, if the beam of charged particles strikes the target 12 at a point corresponding to the center of the target, as illustrated by the solid line 32, then the axis of symmetry 34 of the lobe 36, both of which are illustrated by solid lines, will extend perpendicular to the plane of the target 12. If, however, the beam of charged particles strikes the target 12 at an angle of incidence different from 90 and at a point displaced from the center of the target, as illustrated by the broken line 38, the resulting lobe pattern 40 will be tilted by an amount corresponding to the change in the angle of incidence of the particle beam. This change is illustrated by the change in intensity, designated 0 at the shoulders of the lobe. The broken line 42 indicates the symmetrical axis of the tilted lobe pattern 40.

FIG. 3 illustrates in more detail the ionization chamber 18. More particularly, the ionization chamber includes an arrangement of electrodes for measuring parameters of the radiation lobe pattern in order to detect both changes in the angle of incidence of the particle hem and changes in the point at which the particle beam strikes the target.

The electrodes take the form of four planar electrodes 44, 46, 48, 50, each of which is situated in one of the quadrants of the circular disc-shaped ionization chamber. The electrodes 44, 46, 48, 50, which comprise the inner set of electrodes, are each slightly spaced from a central axis of the disc-shaped chamber 18 and each extends outwardly for a distance of approximately A of the radial distance of the ionization chamber. As illustrated in FIG. 1, the inner quadrant electrodes are positioned under the flattening filter 16 at locations directly beneath the steepest slope of the flattening filter. By so positioning these inner quadrant electrodes, the tilt of the lobe pattern in the region of the flattening filter is compensated for by increased absorption in the flattening filter. Thus, these inner quadrant electrodes are only responsive to changes in the position of the radiation lobe. Each of the inner electrodes 44, 46, 48, 50 is connected to a corresponding one of four output terminals 52, 54, 56, 58.

The ionization chamber 18 also includes an outer set of four planar electrodes 60, 62, 64, 66, each of which is positioned in the same quadrant as one of the inner electrodes 44, 46, 48, 50. The electrodes of the outer set are of an arcuate configuration with the center of curvature of the curved portions thereof being the central axis of the disc-shaped chamber 18. The electrodes of the outer set are radially spaced from the central axis of the ionization chamber at positions more remote than the inner electrodes. The electrodes of the outer set are disposed at locations to detect the intensity of the radiation lobe pattern at the outer edges, or shoulders, of the lobe to thereby measure the tilt of the lobe. Each of the outer electrodes 60, 62, 64, 66 is electrically connected to a corresponding one of four output terminals 68, 70, 72, 74.

The planer electrodes 44, 46, 48, 50, 60, 62, 64, 66 are all placed in a single plane in the ionization chamber 18 and are supported by an insulative plate 67 which is positioned in a disc-shaped housing member. A planar high voltage electrode (not shown) is placed in spaced parallel relationship with the other electrodes, and the chamber 18 is filled with an ionizable gas. Thus, each of the detector electrodes collects ion current proportional to the radiation field intensity averaged over the electrode area. The aforementioned United States Patent Application by R. D. McIntyre provides further details of the construction of the ionization chamber 18.

As illustrated in FIG. 4 the inner electrodes 46, 48 are connected to the input terminals of a differential servoamplifier 76 having its output terminal connected to one of the terminals of a positional error symmetry servomechanism coil 78. The other terminal of this coil 78 is connected directly to ground. Also, a meter 80 is connected between the output terminal of the differential servoamplifier 76 and ground to provide an indication of the value of the compensating signal being applied to the positional servomechanism coil 78.

Similarly, as illustrated in FIG. 5 the other inner electrodes 44, 50 are connected to the input terminals of a differential servoamplifier 82 having its output terminal connected to one of the terminals of another positional error symmetry servomechanism coil 84. The other terminal of this coil 84 is connected directly to ground. Also, the output terminal of the differential servoamplifier 82 is connected through a meter 86 to ground.

Thus, the electrodes 46, 48 serve to monitor along one coordinate axis the displacement of the radiation lobe which is representative of the location at which the particle beam strikes the target 12. The electrodes 44, 50 serve to detect changes in the beam position along a second coordinate axis perpendicular to the first axis. Thus, the signals developed by the electrodes 44, 46, 48, 50, when applied to the positional servomechanism coils 78, 84, primarily correct for changes in the position at which the particle beam strikes the target.

The outer electrodes 62, 64 are connected to the input terminals of another differential servo-amplifier 88 having its output terminal connected to one of the terminals of an angular error symmetry servomechanism coil 90. The other terminal of this coil 90 is connected directly to ground. Also, a meter 92 is connected between the output terminal of the servoamplifier 88 and ground. Similarly, the other outer electrodes 60, 66 are connected to the input terminals of still another differential servoamplifier 94 having its output terminal connected to one of the terminals of another angular error symmetry servomechanism coil 96. The other terminal of this coil 96 is connected directly to ground, and a meter 98 is connected between the output terminal of the differential servoamplifier 94 and ground.

Thus, the outer electrodes 62, 64 are responsive to the large changes in intensity at the shoulders of the lobe pattern. Changes in the intensity of the lobe pattern correspond to changes in the angle of incidence of the particle beam on the target. Thus, the electrical signals developed by the outer electrodes 62, 64, when applied through the differential servoamplifler 88, are used to control the energization of the angular error symmetry servomechanism coil 90. An indication of the angular compensation is provided by the meter 92. The other outer electrodes 60, 66 provide similar compensation of the angular error along an axis perpendicular to the axis of the electrodes 62, 64.

Accordingly, the present invention provides for both angular and positional beam alignment at the point where the particle beam strikes the target. Therefore, with the present invention it is possible to maintain a constant axis of symmetry for the radiation pattern emitted by the target.

Although the invention has been shown in connection with the preferred embodiment, it will be readily apparent to those skilled in the art that various changes in form and arrangement of parts may be made to fit requirements without departing from the spirit and scope of the invention as defined by the appended claims.

Having thus described out invention, we claim:

1. A linear particle accelerator apparatus including means for forming and projecting a beam of charged particles along a substantially linear path, means for deflecting the particle beam, target means disposed in the beam path for emitting radiation upon being struck by charged particles; detector means having first sensor means for developing first signals representative of the position of the point at which the particle beam strikes the target means, and second sensor means for developing second signals representative of the angle of incidence of the particle beam on the target means; and circuit means coupled to said first and second sensor means for presenting output indications representative of both the position of the particle beam and the angle of incidence of the beam on the target means.

2. A linear particle accelerator apparatus as defined in claim 1 including control circuit means coupled between said first and second sensing means and said deflection means for applying control signals to said deflection means in dependence on the value of said first and second signals to cause the particle beam to be deflected in response to a displacement of the position at which the beam strikes the target means and to a change in the angle of incidence of said beam on the target means.

3. An apparatus as defined in claim 1 wherein said emitted radiation has a field pattern which generally takes the form of a lobe configuration extending from said target means, said first sensor means disposed to monitor radiation levels in proximity to the center of said radiation lobe and said second sensor means disposed to monitor radiation levels in proximity with the outer edges of said radiation lobe.

4. An apparatus as defined in claim 3 wherein said first sensor means comprises a plurality of electrodes disposed on opposite sides of an axis of symmetry of said radiation lobe and in proximity with said axis, and said second sensor means comprises a plurality of electrodes disposed on opposite sides of said axis of symmetry and more remote from said axis than said plurality of electrodes of said first sensor means.

5. An apparatus as defined in claim 3 wherein said first sensor means comprises four electrodes disposed in a plane perpendicular to an axis of symmetry of said radiation lobe and spaced from said axis at substantially equal distances, and said second sensor means comprises four electrodes disposed in a plane perpendicular to said axis of symmetry and spaced from said axis at substantially equal distances being greater than the distances of said electrodes of said first sensor means.

6. An apparatus as defined in claim 5 wherein said four electrodes in said first sensor means are each disposed in one of the quadrants of a circular cross section of said radiation lobe taken at right angles to said axis of symmetry.

7. An apparatus as defined in claim 5 wherein said four electrodes in said sensor means are each disposed in one of the quadrants of a circular cross section of said radiation lobe taken at right angles to said axis of symmetry.

8. An apparatus as defined in claim 6 including a beam flattening filter means having a cross section of variable slope disposed between said target means and said detector means, and said four electrodes in said first sensor means disposed to receive substantially one radiation passing through the filter means at points of maximum slope.

9. An apparatus as defined in claim 7 including a beam flattening filter means having a cross section of variable slope disposed between said target means and said detector means, and said four electrodes in said first sensor means disposed to receive substantially one radiation passing through the filter means at points of maximum slope.

10. A system comprising a charged particle accelerator for forming and projecting a beam of charged particles along a path, means for varying the alignment of said beam path, target means disposed in said path, said target means being capable of generating a radiation field upon being struck by said beam, and inner sensor means disposed in said radiation field for monitoring changes in radiation intensity in an inner portion of said field, said changes in said radiation intensity in said inner portion of said field being representative of changes in the position on said target means at which said beam strikes said target means.

11. The system of claim 10 further comprising outer sensor means disposed in said radiation field for monitoring changes in radiation intensity in an outer portion of said field, said changes in said radiation intensity in said outer portion of said field being representative of changes in the angle of incidence of said beam on said target means.

12. The system of claim 11 wherein said inner sensor means and said outer sensor means comprise planar electrodes, said electrodes all being coplanar with each other.

13. The system of claim 11 wherein said inner sensor means comprises an inner set of radiation responsive elements disposed symmetrically about and spaced apart from an axis and lying in a plane perpendicular to said axis, and said outer sensor means comprises an outer set of radiation responsive elements disposed symmetrically about and spaced apart from said axis and lying in said plane, the radiation responsive elements of said outer set being spaced further apart from said axis than the radiation responsive elements of said inner set.

14. The system of claim 13 wherein said inner set of radiation responsive elements comprises four planar electrodes disposed in a quadrature relationship with respect to each other about said axis, and said outer set of radiation responsive elements comprises four planar electrodes disposed in a quadrature relationship with respect to each other about said axis, each of said planar electrodes comprising a sheet of electrically conductive material secured to a planar surface on an electrically insulating electrode support structure.

15. The system of claim 14 wherein each of said planar electrodes is secured to said planar surface of said insulating electrode support structure by a vacuum deposition bond.

16. The system of claim 11 wherein said inner sensor means and said outer sensor means are disposed in an ionization chamber, said ionization chamber beig disposed in said radiation field.

17. The system of claim 16 wherein said ionization chamber further comprises a hermetically sealed housing member containing an ionizable gas, said inner sensor means and said outer sensor means each comprising a plurality of planar electrodes, each of said electrodes of said inner sensor means and said outer sensor means comprisng a sheet of electrically conductive material secured to a planar surface of an electrically insulating electrode support structure disposed within said housing member.

18. The system of claim 17 wherein said inner sensor means comprises four electrodes arranged in a quadrature relationship with respect to each other in a disposition symmetrical about and spaced apart from an axis perpendicular to the planar surface of said electrode support structure, and said outer sensor means comprises four electrodes arranged in a quadrature relationship with respect to each other in a disposition symmetrical about and spaced apart from said axis, the electrodes of said outer sensor means being spaced further apart from said axis than the electrodes of said inner sensor means.

19. The system of claim 10 further comprising a flattening filter disposed in said radiation field intermediate said target means and said inner sensor means.

20. The system of claim 19 wherein said inner sensor means comprises radiation responsive elements disposed in said radiation field in alignment with the steepest slope of said flattening filter.

21. A system comprising a charged particle accelerator for forming and projecting a beam of charged particles along a path, means for varying the alignment of said beam path, target means disposed in said path, said target means being capable of generating a radiation field upon being struck by said beam, and outer sensor means disposed in said radiation field for monitoring changes in radiation intensity in an outer portion of said field, said changes in said radiation intensity in said outer portion of said field being representative of changes in the angle of incidence of said beam on said target means.

22. The system of claim 21 further comprising inner sensor means disposed in said radiation field for monitoring changes in radiation intensity in an inner portion of said field, said changes in said radiation intensity in said inner portion of said field being representative of changes in the position on said target means at which said beam strikes said target means.

23. The system of claim 21 wherein said outer sensor means comprises four electrodes of arcuate configuration arranged in a quadrature relationship with respect to each other in a coplanar disposition symmetrical about and spaced apart from an axis perpendicular to said plane.

24. The system of claim 22 further comprising circuit means for coupling said inner and outer sensor means to said means for varying the alignment of said beam path, whereby said inner and outer sensor means can generate control signals and apply said control signals to said means for varying the alignment of said beam path in response to said monitored changes in radiation intensity in said inner and outer portions of said radiation field, thereby causing said beam to maintain a constant angle of incidence upon said target means at a constant position on said target means.

25. The method of continuously monitoring the orientation of a radiation field produced by the bombardment of a beam of charged particles upon a target means, said method comprising the steps of locating first sensor means in said field, said first sensor means being responsive to changes in the position on said target means at which said beam strikes said target means, and locating second sensor means in said field, said second sensor means being responsive to changes in the angle of incidence of said beam on said target means. 

1. A linear particle accelerator apparatus including means for forming and projecting a beam of charged particles along a substantially linear path, means for deflecting the particle beam, target means disposed in the beam path for emitting radiation upon being struck by charged particles; detector means having first sensor means for developing first signals representative of the position of the point at which the particle beam strikes the target means, and second sensor means for developing second signals representative of the angle of incidence of the particle beam on the target means; and circuit means coupled to said first and second sensor means for presenting output indications representative of both the position of the particle beam and the angle of incidence of the beam on the target means.
 2. A linear particle accelerator apparatus as defined in claim 1 including control circuit means coupled between said first and second sensing means and said deflection means for applying control signals to said deflection means in dependence on the value of said first and second signals to cause the particle beam to be deflected in response to a displacement of the position at which the beam strikes the target means and to a change in the angle of incidence of said beam on the target means.
 3. An apparatus as defined in claim 1 wherein said emitted radiation has a field pattern which generally takes the form of a lobe configuration extending from said target means, said first sensor means disposed to monitor radiation levels in proximity to the center of said radiation lobe and said second sensor means disposed to monitor radiation levels in proximity with the outer edges of said radiation lobe.
 4. An apparatus as defined in claim 3 wherein said first sensor means comprises a plurality of electrodes disposed on opposite sides of an axis of symmetry of said radiation lobe and in proximity with said axis, and said second sensor means comprises a plurality of electrodes disposed on opposite sides of said axis of symmetry and more remote from said axis than said plurality of electrodes of said first sensor means.
 5. An apparatus as defined in claim 3 wherein said first sensor means comprises four electrodes disposed in a plane perpendicular to an axis of symmetry of said radiation lobe and spaced from said axis at substantially equal distances, and said second sensor means comprises four electrodes disposed in a plane perpendicular to said axis of symmetry and spaced from said axis at substantially equal distances being greater than the distances of said electrodes of said first sensor means.
 6. An apparatus as defined in claim 5 wherein said four electrodes in said first sensor means are each disposed in one of the quadrants of a circular cross section of said radiation lobe taken at right angles to said axis of symmetry.
 7. An apparatus as defined in claim 5 wherein said four electrodes in said sensor means are each disposed in one of the quadrants of a circular cross section of said radiation lobe taken at right angles to said axis of symmetry.
 8. An apparatus as defined in claim 6 including a beam flattening filter means having a cross section of variable slope disposed between said target means and said detector means, and Said four electrodes in said first sensor means disposed to receive substantially one radiation passing through the filter means at points of maximum slope.
 9. An apparatus as defined in claim 7 including a beam flattening filter means having a cross section of variable slope disposed between said target means and said detector means, and said four electrodes in said first sensor means disposed to receive substantially one radiation passing through the filter means at points of maximum slope.
 10. A system comprising a charged particle accelerator for forming and projecting a beam of charged particles along a path, means for varying the alignment of said beam path, target means disposed in said path, said target means being capable of generating a radiation field upon being struck by said beam, and inner sensor means disposed in said radiation field for monitoring changes in radiation intensity in an inner portion of said field, said changes in said radiation intensity in said inner portion of said field being representative of changes in the position on said target means at which said beam strikes said target means.
 11. The system of claim 10 further comprising outer sensor means disposed in said radiation field for monitoring changes in radiation intensity in an outer portion of said field, said changes in said radiation intensity in said outer portion of said field being representative of changes in the angle of incidence of said beam on said target means.
 12. The system of claim 11 wherein said inner sensor means and said outer sensor means comprise planar electrodes, said electrodes all being coplanar with each other.
 13. The system of claim 11 wherein said inner sensor means comprises an inner set of radiation responsive elements disposed symmetrically about and spaced apart from an axis and lying in a plane perpendicular to said axis, and said outer sensor means comprises an outer set of radiation responsive elements disposed symmetrically about and spaced apart from said axis and lying in said plane, the radiation responsive elements of said outer set being spaced further apart from said axis than the radiation responsive elements of said inner set.
 14. The system of claim 13 wherein said inner set of radiation responsive elements comprises four planar electrodes disposed in a quadrature relationship with respect to each other about said axis, and said outer set of radiation responsive elements comprises four planar electrodes disposed in a quadrature relationship with respect to each other about said axis, each of said planar electrodes comprising a sheet of electrically conductive material secured to a planar surface on an electrically insulating electrode support structure.
 15. The system of claim 14 wherein each of said planar electrodes is secured to said planar surface of said insulating electrode support structure by a vacuum deposition bond.
 16. The system of claim 11 wherein said inner sensor means and said outer sensor means are disposed in an ionization chamber, said ionization chamber beig disposed in said radiation field.
 17. The system of claim 16 wherein said ionization chamber further comprises a hermetically sealed housing member containing an ionizable gas, said inner sensor means and said outer sensor means each comprising a plurality of planar electrodes, each of said electrodes of said inner sensor means and said outer sensor means comprisng a sheet of electrically conductive material secured to a planar surface of an electrically insulating electrode support structure disposed within said housing member.
 18. The system of claim 17 wherein said inner sensor means comprises four electrodes arranged in a quadrature relationship with respect to each other in a disposition symmetrical about and spaced apart from an axis perpendicular to the planar surface of said electrode support structure, and said outer sensor means comprises four electrodes arranged in a quadrature relationship with respect to each other in a dIsposition symmetrical about and spaced apart from said axis, the electrodes of said outer sensor means being spaced further apart from said axis than the electrodes of said inner sensor means.
 19. The system of claim 10 further comprising a flattening filter disposed in said radiation field intermediate said target means and said inner sensor means.
 20. The system of claim 19 wherein said inner sensor means comprises radiation responsive elements disposed in said radiation field in alignment with the steepest slope of said flattening filter.
 21. A system comprising a charged particle accelerator for forming and projecting a beam of charged particles along a path, means for varying the alignment of said beam path, target means disposed in said path, said target means being capable of generating a radiation field upon being struck by said beam, and outer sensor means disposed in said radiation field for monitoring changes in radiation intensity in an outer portion of said field, said changes in said radiation intensity in said outer portion of said field being representative of changes in the angle of incidence of said beam on said target means.
 22. The system of claim 21 further comprising inner sensor means disposed in said radiation field for monitoring changes in radiation intensity in an inner portion of said field, said changes in said radiation intensity in said inner portion of said field being representative of changes in the position on said target means at which said beam strikes said target means.
 23. The system of claim 21 wherein said outer sensor means comprises four electrodes of arcuate configuration arranged in a quadrature relationship with respect to each other in a coplanar disposition symmetrical about and spaced apart from an axis perpendicular to said plane.
 24. The system of claim 22 further comprising circuit means for coupling said inner and outer sensor means to said means for varying the alignment of said beam path, whereby said inner and outer sensor means can generate control signals and apply said control signals to said means for varying the alignment of said beam path in response to said monitored changes in radiation intensity in said inner and outer portions of said radiation field, thereby causing said beam to maintain a constant angle of incidence upon said target means at a constant position on said target means.
 25. The method of continuously monitoring the orientation of a radiation field produced by the bombardment of a beam of charged particles upon a target means, said method comprising the steps of locating first sensor means in said field, said first sensor means being responsive to changes in the position on said target means at which said beam strikes said target means, and locating second sensor means in said field, said second sensor means being responsive to changes in the angle of incidence of said beam on said target means. 